As the complexity of integrated circuits continues to grow, improved packaging technologies have been developed to keep pace with the increasing circuit densities and the increasing numbers of interconnects. For example, one of these techniques, known as embedded wafer level ball grid array (eWLB), was developed to support the fan-out of densely spaced integrated-circuit terminals to the solder ball interconnects of a ball grid array (BGA) package.
With eWLB packaging, a processed wafer is diced and the diced chips are spaced apart on a carrier. The spaces between the chips are filled with a molding compound, which is then cured to form an artificial wafer. Thin-film technology is used to form one or more redistribution layers, which connect the pads of the integrated circuit to the package interconnects, e.g., the solder balls. The redistribution layers, which can include one or more conductive layers and intervening dielectric layers, vias between conductive layers, and the like, allow for a flexible and efficient fan-out of the integrated-circuit inputs and outputs to the packaging interconnects. U.S. Pat. No. 8,237,259 B2, titled “Embedded Chip Package” and issued 7 Aug. 2012, provides details of the eWLB packaging technique; the entire contents of the foregoing patent are incorporated herein by reference.
Radio-frequency (RF) integrated circuits (RFICs) include circuit elements that produce or operate on signals in the radio-frequency range, which, according to some definitions, extends from about a few kilohertz to 300 gigahertz (GHz) or more. It will be appreciated that that frequencies between about 1 GHz and 300 GHz are often referred to as microwave frequencies. For the purposes of this disclosure, however, the term “radio-frequency” (or RF) is used to refer broadly to signals ranging in frequency from a few megahertz (MHz) to 100 GHz or more, and more particularly to refer to signals that are typically carried from one place to another by transmission-line and/or waveguide structures that are specifically designed for the propagation of high-frequency electromagnetic waves.
RF integrated circuits create additional challenges for packaging, particularly as signal frequencies increase. RF interconnects that extend for more than very short distances are most efficiently realized using transmission line structures, such as stripline, co-planar waveguide, or microstrip structures, or waveguide structures, such as rectangular waveguides. When RF circuits are mounted on planar substrates such as printed circuit boards, for instance, they are usually coupled directly to planar transmission lines that can be easily fabricated on the same substrate or printed circuit board.
Especially at higher frequencies, it may be preferable to couple an RF circuit to a rectangular waveguides. A variety of transition element structures have been developed to couple transmission lines formed on circuit boards, e.g., microstrip, co-planar waveguide, or stripline transmission lines, to rectangular waveguides. Many of these transition elements use a vertical probe, pin or vertical small antenna to excite the electric (E) field of an electromagnetic field being transitioned from the co-planar waveguide to the rectangular waveguide. This is probe changes the propagation mode of the electromagnetic wave from the TEM mode which is used by co-planar waveguides to a transverse electromagnetic (TE) mode, such as TE10, which can be propagated by a rectangular waveguide.
The use of vertical elements like these probes can create fabrication difficulties. The fabrication of vertical pins, probes or antennas is more complicated than the fabrication of planar structures, and usually can only be done at a lower metal resolution (e.g., larger metal pitch and diameter) in comparison to horizontal or planar structures fabricated on planar substrates. Accordingly, improved transition structures that are more compatible with advanced integrated-circuit packaging techniques are needed.